Aldol Condensations Catalyzed by Hydrotalcites

Aldolization is widely used in organic chemistry, because it enables the creation of C-C bonds. It is commonly used to manufacture solvents (diacetone alcohol, iso-phorone), intermediates for the manufacture of perfumes and pharmaceuticals (chalcones and more generally a,P unsaturated ketones), and plasticizers. This reversible reaction can be catalyzed by acids or bases [1], but basic catalysis is usually preferred. The substitution of the homogeneous bases by solids in the process is environmentally highly desirable. 

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Lakshmi Kantam, M., Choudary, B. M., Reddy, C. V., Koteswara Rao, K. and Figueras, F. Aldol and Knoevenagel condensations catalyzed by modified Mg-Al hydrotalcite a solid base as catalyst useful in synthetic organic chemistry, Chem. Commun., 1998, 1033-1034. 

Figure 2.42 Examples of reactions catalyzed by hydrotalcite-based catalysts for the synthesis of flavors and fragrances (a) aldol condensation (b) Knoevenagel condensation (c) double-bond isomerization.

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

The first step in the base-catalyzed condensation is the proton abstraction of active methylene groups with base catalysts, followed by attack of the anion on a carbonyl moiety. Elimination of water then produces the CMD double bond. Numerous heterogeneous catalysts have been reported to promote these condensations, including the rehydrated hydrotalcites described above for the Aldol reactions (41,51), vanadate apatite (52), and amine-functionalized silica (53). The merits of using these heterogeneous catalysts are the ease of product isolation and catalyst reusability, which is associated with elimination of the necessity to neutralize homogeneous bases such as NaOH and NaOMe. 

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